Chapter 11 Cyclonic Storms
The smallest cyclonic storm is a thunderstorm and the largest is a hurricane or typhoon.
The most effective learning of hurricanes/typhoons begins with effective learning of
thunderstorms and tornadoes. So that‘s where we begin!
All thunderstorms ( such as the one of Fi) require three
ingredients for their formation:
A lifting mechanism.
2.1 Sources of moisture
Typical sources of moisture are large bodies of water such as the
Atlantic and Pacific oceans as well as the Gulf of Mexico and
the Great Lakes (Fig.2). The southeastern U.S. and especially
Florida has access to two moisture sources in the Atlantic ocean
and the Gulf of Mexico which helps explain why there are so many thunderstorms in that
region. Here in Southwestern Ontario, the Great Lakes supply the moisture.
Air is considered unstable if it continues to rise when given a nudge upward (or continues
to sink if given a nudge downward). An unstable air mass is
characterized by warm moist air near the surface and cold dry air
above. In these situations, if a bubble or parcel of air is forced
upward it will continue to rise on its own. As it rises it cools and
some of the water vapor will condense, forming the familiar tall
cumulonimbus cloud that is the thunderstorm. Air that is forced
upward will continue to rise, and air that is forced downward will
continue to sink.
2.3 Sources of Lift
Typically, for a thunderstorm to develop there needs to be a mechanism which initiates
the upward motion, something that will give the air a nudge upward. This is done by
This heating of the ground and lower atmosphere is not uniform. For example, a grassy
field will heat at a slower rate than a paved street or bare rock. The warmest air, called
thermals, tends to rise.
Cold Fronts Fronts are the boundary between two air masses of different temperatures. Fronts lift
warm moist air. Cold fronts lift air the most abruptly. If the air is moist and unstable
thunderstorms will form along the cold front. As I‘m writing this, there‘s a mass of warm, moist
air pouring into the (cooler) London area from the southern USA, and thunderstorms are springing up all
along the fro.t
As air encounters a mountain it is forced up the slope of the terrain. Upslope
thunderstorms are common in the Rocky Mountain west during the summer.
2.4 Life Cycle of a Thunderstorm
The building block of all thunderstorms is the thunderstorm cell. The thunderstorm cell
has a distinct life-cycle that lasts about 30 minutes.
The first stage is the towering cumulus stage. A cumulus cloud begins to grow
vertically, perhaps to a height of 6 km. Air within the cloud is dominated by updraft with
some turbulent eddies around the edges.
The second stage is the mature stage. The storm has considerable depth, often reaching
12 to 18 km. Strong updrafts and downdrafts coexist. The updrafts at this stage can be
extremely strong: the stronger the updraft, the more weight of rain and hail that can be
supported. This is the most dangerous stage ( see section on hazards described bel).
The final stage is the dissipating stage. The downdraft cuts off the updraft. The storm no
longer has a supply of warm moist air to maintain itself and therefore it dissipates. Light
rain and weak outflow winds may remain for a while during this stage, before leaving
behind just a remnant anvil-shaped top.
2.5 Thunderstorm Types
Although there are times when a thunderstorm
consists of just one ordinary cell that transitions through its life cycle and dissipates without additional new cell formation, much more
commonly thunderstorms form in
clusters (Fig.7) with numerous cells in
various stages of development merging
together. Unlike ordinary single cells,
cluster storms can last for several hours
producing large hail, damaging winds,
flash flooding, and isolated tornados.
Sometimes thunderstorms will form in a
line which can extend laterally for hundreds of miles. These "squall lines" (Fig.8) can
persist for many hours and produce damaging winds and hail. The rain cooled air or "gust
front" spreading out from underneath the squall line acts as a mini cold front, continually
lifting warm moist air to fuel the storms. If the cells in the squall line are producing
down-blast winds exceeding 93km/h and the length of the squall line exceeds 450 km, the
result is termed a ‗derecho‘ – not nice!
Supercell thunderstorms are a special kind of single cell thunderstorm that can persist for
many hours. All the individual cells unite, producing one gigantic rotating cyclonic
storm. Figure 9 shows one supercell covering the whole of Oklahoma State. They are
responsible for nearly all of the significant tornados produced in the U.S. and for most of
the hailstones larger than golf ball size. Supercells are also known to produce extreme
winds and flash flooding. The most ideal conditions for supercells occur when the winds
are veering or turning clockwise with height. Beneath the supercell, the rotation of the
storm is often visible as well. The lowering in Figure 10 represents the wall cloud. The
wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would
usually do so within the wall cloud.
2.6 Thunderstorm Hazards
Lightning, hail and erratic strong winds are the obvious hazards associated with
thunderstorms; for example, in any year in North America, more people are killed by
lightning than by tornadoes and hurricanes combined. Nevertheless, we have a great deal of content to cover in this course, so we‘re going to skip any further consideration of
If you wish to read about these hazards – for your own interests – the following sites are
A tornado is the smallest, most violent weather
disturbance that occurs on Earth (Fig.11).A
tornado is a violently rotating usually
counterclockwise in the northern hemisp) column of
air descending from a thunderstorm and in contact
with the ground. Although tornados are usually
Fig. 11 brief, lasting only a few minutes, they can
sometimes last for more than an hour and travel
several kilometers causing considerable damage.
In a typical year about 1000 tornados will strike the
United States. The peak of the tornado season is
April through June and more tornados strike the
central United States than any other place in the
world. This area has been nicknamed "tornado alley"
Most tornados are spawned from supercell
Fig.12 thunderstorms. Supercell thunderstorms are
characterized by a persistent rotating updraft and form in environments of strong vertical
wind shear. Wind shear is the change in wind speed and/or direction with height.
Fig.12 Figure 13 shows the creation of a ‗tube by wind shear. The
updraft of the thunderstorm
Fig.13 then lifts the rotating column Fig.14
of air (Fig.14) created by the
speed shear. This provides
two different rotations:
cyclonic (or counter
clockwise rotation) and anti-
cyclonic (or clockwise
rotation). In the Northern Hemisphere the directional shear amplifies the cyclonic rotation
and diminishes the anti-cyclonic rotation (the rotation on the right side of the updraft in
the illustration at left). A more or less vertical tube (whose walls consist of moisture) is
then rotating rapidly, and establishing an internal vacuum. As the funnel descends the
water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel.
The descending funnel is made visible because of the water droplets. The funnel takes on
the color of the cloud droplets, which is white. Due to the air movement, dust and debris
on the ground will begin rotating, often becoming a few meters high and hundreds of
After the funnel touches the ground (thereby officially becoming a tornado), the color of
the funnel will change. The color often depends upon the type of dirt and debris moved
over (red dirt produces a red tornado, black dirt a black tornado.) (Fig.15). At the center of the
tornado, the vortex is open and the air is clear, not too unlike a hurricane.
Fig.15 Tornados can last from several seconds to more than an hour but
most last less than 10 minutes. The size and/or shape of a tornado
is no measure of its strength. Occasionally, small tornados do
major damage and some very large tornados, over a quarter-mile
wide, have produced only light damage.
Canada is second only to the US for numbers of tornadoes; the
whole country experiences roughly 100 in any one year. How does
Southern Ontario compare for tornado frequency? It was stated in
a university study that Southern Ontario has about half of the
number of tornados per 10,000 km as the most afflicted region of
the U.S. in Oklahoma and Kansas. The Red River valley of southern Manitoba has
reported frequencies about one-third of the highest values in the U.S. Southern Canada
definitely has a tornado hazard, all the way from New Brunswick to the B.C. interior.
Only in extreme southwestern Ontario, though, the average is about 28 tornado sightings
per year. In Ontario, tornadoes usually occur in the late afternoon or early evening during
the months of May, June or July. From one storm line in one day at the end of July 2006,
8 tornadoes touched down in southern Ontario (highest rating: 2 on Fujita Sc.le)
If you want to look at some spectacular photo sequences of tornadoes, take a look at the
following site (it's the best tornado photo site I know of): http://www.squidoo.com/the-
Classification of tornadoes is by the Fujita Scale (Table 1, below):
Category Category Level Of Damage
F-0 Gale Tornado Chimneys damaged; branches broken off trees; shallow-rooted trees uprooted; sign
40 - 72 MPH boards damaged.
F-1 Moderate Roof surfaces peeled off; mobile homes pushed off foundations or overturned; moving
Tornado autos pushed off roads.
73 - 112 MPH
F-2 Significant Roofs torn off frame houses; mobile homes demolished; box cars pushed over; large
113 - 157 MPH trees snapped or uprooted; light-object projectiles generated.
F-3 Severe Roofs and some walls torn off well-constructed houses; trains overturned; most trees Tornado in forest uprooted; heavy cars lifted off the ground and thrown.
158 - 206 MPH
F-4 Devastating Well-constructed houses leveled; structures with weak foundations relocated; cars
Tornado thrown and large projectiles generated.
207 - 260 MPH
F-5 Incredible Strong frame houses lifted off foundations and carried considerable distance to
Tornado disintegrate; automobile-sized projectiles hurtle through the air in excess of 100 yards;
261 - 318 MPH trees debarked; other incredible phenomena expected.
4.0 Hurricanes and Other Cyclonic Monsters
The above section about thunderstorms and tornadoes is just an introduction to this stuff!
A hurricane is a severe tropical storm that forms in the North Atlantic Ocean, the
Ocean east of the
dateline, or the
South Pacific Ocean
east of 160E.
and light winds
above them. If the
right conditions last
long enough, a
Fig.16 hurricane can
rains and floods.
If I ask for a definition of ‗hurricanes‘, this is what I want:
severe cyclonic tropical storm in the North Atlantic Basin o o
originates within the belt of tropical trade winds, roughly from 5 to 20 from the
rotates counterclockwise around an ‗eye‘ with minimum wind speeds of 119
In other regions of the world, these types of storms have different names:
Typhoon — ( the Northwest Pacific Ocean west of the date)ine
Severe Tropical Cyclone — ( the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean
east of 90)
Severe Cyclonic Storm — ( the North Indian Ocea)
Tropical Cyclone — ( the Southwest Indian Ocea)
They all originate only in the tropical trade winds where the temperatures are warm;
Figure 16 shows the distribution of big storms for various regions of the world. Note the general restriction to latitudes of about 5 to 20 ; any guesses as to why that might be (think
Fig.17 There are on average six Atlantic hurricanes each year.
The Atlantic hurricane season begins June 1 and ends
November 30. The East Pacific hurricane season runs
from May 15 through November 30, with peak activity
occurring during July through September. In a normal
season, the East Pacific would expect 15 or 16 tropical
storms. Nine of these would become hurricanes, of
which four or five would be major hurricanes (Fig.17
shows the cyclonic disturbances of one day; obviously,
not all developed into big storms).
When hurricanes move onto land, the heavy rain, strong winds
and heavy waves can damage buildings, trees and cars. The
heavy waves are called a storm surge. Storm surge is very
dangerous and a major reason why you MUST stay away from
the ocean during a hurricane.
What is increasingly alarming is the cost of these events – both in
human life and in money. Why is that? It‘s exactly the same
reason that we mentioned in Chapter 1: far too many
governments allow people to inhabit land that is doomed to
recurring storm damage.
In the case of tropical cyclones, hurricanes or typhoons, the
physical movements that give birth to the storms are the
following (you can follow the sequence with Figure 18):A slowly
advancing or even a stationary cold front develops a bulge (or
Fig.18 'wave') in the boundary between cool and warm air moving in
The bulge or wave grows as the moving air is deflected, forming a warm front moving
right on the right side and a cool front moving left on the left side. Cold air is denser than
warm air, and the cold air moves faster than the slower warm front. As the faster moving
cold air catches up with the slower moving warm air, the cold air under-rides the warm
air, lifting a ‗cell‘ upward.
This lifting action produces a low-pressure area at the point where the two fronts come
together. The lifted air expands, cools adiabatically, and reaches the dew point. First
clouds develop over the low-pressure area.
When the cold front has completely overtaken the warm front, an occlusion is formed.
An occluded front is one that has been lifted completely off the ground into the
atmosphere. The disturbance now is a fully developed cyclonic storm, with a fully developed low-
pressure center, and moving generally with the easterly winds. [Cyclonic storms which
strike the eastern coast about Florida normally have begun in Africa and developed into
cyclonic storms only after experiencing the warm waters of the tropical Atlantic. They
have been sent westward by the northeast trade winds.].
4.2 Origin of Hurricanes
Hurricanes (northern hemisphere) begin with a disturbance in the westward-flowing air
not far from the equator (Figure 19:
This is a sea surface temperature map
for the northern hemisphere summer.
The yellow, orange, and red colours
show water temperatures warm enough
to sustain hurricanes (> 26.5°C)). The
sea surface temperature must be 26 Co
Fig.19 or higher (and so you see the implication of
global warming?). As the water vapor rises, according to the above sequence, it condenses,
which releases a very large amount of latent heat energy - the release of latent heat in
the transformation from water vapor to water liquid. This is the main energy source of a
hurricane. In a single day, an average hurricane will release latent heat energy equivalent
to about 1.6 x10 kilowatt-hours; that is about 8000 x more than the ele